Some types of concrete tubing such as those intended for use as sewer pipe are provided with plastic lining for protecting the concrete from the effects of material in the pipe. In exemplary embodiments the lining is polyvinyl chloride or other thermoplastic material in the order of two to five millimeters thick. Such tubing may be lined around the entire circumference or around an upper portion of the circumference. The latter embodiment is suitable for sewer pipe where a lower portion of the pipe is protected by immersion in liquid contents of the pipe and only an upper portion is exposed to gases that could damage the concrete.
In either embodiment it is important to seal the plastic lining within adjacent sections of tubing at the butt joint between the sections. In some embodiments a flap of extra plastic is provided at one end of each section of tubing and that flap is lapped over the plastic lining in an adjacent section of pipe and sealed in place. Alternatively, a strip of plastic can be added along the joint overlapping the plastic lining in adjacent sections of tubing. In either embodiment the overlapped portion is preferably heat welded for providing a fluid-tight seal.
It has been the practice to manually weld such plastic linings using a hot air gun or the like for melting the plastic. Such techniques are costly and difficult and can be subject to variable quality. It is therefore desirable to provide a technique for rapidly and reliably welding the joints in plastic lining in concrete tubing or the like. Preferably the technique should be suitable for tubes of a variety of diameters and for either lap joints employing a flap of plastic from one tubing section overlapping the plastic in another section or joints employing an added sealing strip of plastic over the lining in adjacent sections. Preferably the technique is suitable for welding the lining adjacent a butt joint normal to the tubing axis or at a beveled or mitered joint adjacent a change in direction of the tubing.